V/Q (ventilation/perfusion) scan is a scintigraphic examination of the lung that evaluates pulmonary vasculature perfusion and segmental bronchoalveolar tree ventilation.
- diagnosis of suspected pulmonary embolism (PE)
- monitor pulmonary function following lung transplant
- provide preoperative estimates of lung function in lung cancer patients, where pneumonectomy is planned
The ventilation scan performed using radio-isotope labeled aerosols like technetium-99m DTPA delivered to the patient through a non-rebreathing mask, with the patient supine. The micro-aerosol particles are small enough to reach the distal tracheobronchial tree and reflect regional ventilation 1. The patient is then imaged in the upright position in three phases: initial breath, equilibrium and washout.
The perfusion lung scan involves injecting Tc-99m MAA intravenously in the supine position. The MAA particles are just small enough to get lodged in the pre-capillary arterioles. A high resolution, large field of view gamma camera is used to image the lungs.
Modified PIOPED criteria can be used to determine the probability of a PE on a V/Q scan. For the diagnosis of PE, a V/Q scan has a reported sensitivity of 77.4% and specificity of 97.7% when compared to CT pulmonary angiography (CTPA) or digital subtraction angiography (DSA) 3.
A normal ventilation scan (even distribution of radionuclide throughout the lung fields) with multiple, bilateral perfusion defects are the classical findings of pulmonary embolism 1.
Causes of ventilation/perfusion mismatch include:
- 1. Brant WE, Helms C. Fundamentals of Diagnostic Radiology. LWW. ISBN:1608319113. Read it at Google Books - Find it at Amazon
- 2. Parker JA, Coleman RE, Grady E et-al. SNM practice guideline for lung scintigraphy 4.0. J Nucl Med Technol. 2012;40 (1): 57-65. doi:10.2967/jnmt.111.101386 - Pubmed citation
- 3. Sostman HD, Stein PD, Gottschalk A, Matta F, Hull R, Goodman L. Acute pulmonary embolism: sensitivity and specificity of ventilation-perfusion scintigraphy in PIOPED II study. (2008) Radiology. 246 (3): 941-6. doi:10.1148/radiol.2463070270 - Pubmed